Another Apple-to-switch-Macs-to-ARM post. "Apple engineers have grown confident that the chip designs used for its mobile devices will one day be powerful enough to run its desktops and laptops, said three people with knowledge of the work, who asked to remain anonymous because the plans are confidential. Apple began using Intel chips for Macs in 2005." No idea when Apple will make the switch, but they will do it. I'm thinking 5-10 year timeframe.

Apple have probably been 'playing' with this for several years. By playing, a small team somewhere has been given a brief to investigate using ARM cpu's in the OSX Product line.
They will have gone away and have been trying all sorts of permutations. They may have even got their colleagues in PA-Semi to create some custom silicon for them.
When the time comes for the project to become mainstream the results of this 'playing' will be fed into their product direction. The 'playing' will have covered all sorts of different ways/permutations etc on how to do this.
They might even say, 'sorry not possible yet because...'
This is what any company that is serious about making this sort of transition would do. Apple have done it before. They might even have several teams looking into it. you can be sure that if/when Apple makes this move they will have a really good idea about what will be produced and sold to the consumers.
This is how new Cars are produced. You see lots of concepts at the motor shows. Very few of them see the light of day but often bits of these concepts appear in new cars a few years later.
Naturally this is all speculation but if I were an apple shareholder(I'm not) I would be expecting them to be doing this sort of thing.